With an ever increasing demand for petroleum, there has been growing interest in renewable feedstocks for manufacturing bioethanol (1). Based on recent economic analysis, a modern biorefinery will utilize about 2000 tons/day of lignocellulosic biomass (“biomass”) for producing biofuels and biochemicals (2). Lignocellulosic fibers comprise a complex network of cellulose, hemicellulose and lignin (3-4) producing a compact matrix, that is difficult to hydrolyze due to poor enzyme accessibility. To improve accessibility of enzymes to the interwoven polysaccharides, a thermochemical treatment (i.e., a “pretreatment”) is typically necessary before enzymatic hydrolysis.
Different types of feed stocks are readily available for making biofuels. Such feedstocks include agricultural residues, woody biomass, municipal waste, oilseeds/cakes and sea weeds. Commercially available oil seed cakes include canola, sunflower, sesame, peanut, palm oil, Jatropha and soybean. At present these different agricultural residues and oil cakes are either used as animal feed, biocompost materials or are land filled. Grasses and oilseed cakes/meals are rich in protein, fiber and other nutrients. It might be possible to utilize the fiber rich portion of the feed stock to make bioethanol by utilizing a suitable thermochemical pretreatment, enzymatic hydrolysis and fermentation process. Economical pretreatment of feed stocks in a continuous manner is quite challenging. For several leading pretreatment processes such as dilute acid, concentrated ammonia (AFEX)™ (hereinafter “AFEX”), steam explosion, and organosolv, a detailed economic analysis has been reported (5).
A wide variety of methods (e.g., concentrated or dilute acids or bases, high temperatures, radiation of various forms) have been used to pretreat lignocellulosic biomass to extract structural carbohydrates to be used to obtain monosaccharides for many different uses. The goal of these pretreatments is to increase the rate and/or yield at which the monosaccharides are subsequently obtained from the structural carbohydrates by chemical or biochemical means, such as acid catalysis, enzymatic catalysis, fermentation or animal digestion. In general, these pretreatments have fallen short of desired economic and technical performance for several reasons: 1) many pretreatments degrade some of the sugars, e.g., to acids or aldehydes, thus reducing yields and inhibiting subsequent biological conversion of the remaining sugars; 2) when chemicals are used in the pretreatment, it is frequently difficult to recover these chemicals at reasonable cost; 3) residual chemicals can negatively affect downstream conversion operations; and 4) the effectiveness of many pretreatments is limited so that the ultimate conversions of structural carbohydrates obtained, independent of lost yield by sugar degradation reactions, is inadequate for competitive process economics. Thus, there are many prior art methods, and they have numerous drawbacks, including those outlined above.
Sufficiently inexpensive monosaccharides from renewable plant biomass can become the basis of chemical and fuels industries, replacing or substituting for petroleum and other fossil feedstocks. Highly reactive lignocellulosic biomass can also become the basis of improved animal feeds, particularly for ruminant animals. Effective, economical pretreatments are required to make these monosaccharides available at high yield and acceptable cost.
The prior art in the pretreatment of plant biomass with anhydrous liquid ammonia or ammonium hydroxide solutions is extensive. Illustrative are the following patents and literature references: U.S. Pat. No. 4,600,590 to Dale; U.S. Pat. No. 4,644,060 to Chou; U.S. Pat. No. 5,037,663 to Dale; U.S. Pat. No. 5,171,592 to Holtzapple et al.; U.S. Pat. No. 5,865,898 to Holtzapple et al.; U.S. Pat. No. 5,939,544 to Karsents et al.; U.S. Pat. No. 5,473,061 to Bredereck et al.; U.S. Pat. No. 6,416,621 to Karstens; U.S. Pat. No. 6,106,888 to Dale et al; U.S. Pat. No. 6,176,176 to Dale et al; U.S. Patent Application No. 2007/0031918, filed Apr. 12, 2006; Felix, A., et al., Anim. Prod. 51 47-61 (1990); and Waiss, A. C., Jr., et al., Journal of Animal Science 35 No. 1, 109-112 (1972). All of these patents and publications are incorporated herein in their entireties.
Ammonia fiber expansion (AFEX) is a leading alkaline pretreatment process that modifies the cell wall ultra-structure without physically extracting lignin and hemicellulose into a separate liquid stream. In addition, the inhibitory compounds formed during the ammonia pretreatment process are insignificant compared to dilute acid pretreatment which play an important inhibitory role during downstream biological processing. The primary advantage of using ammonia during pretreatment is relatively easy recovery and reusability of ammonia due to its high volatility. Close inspection of various ammonia based pretreatments, reveal that ammonia was either used in its liquid state (30-99% ammonia concentration) (6-11), supercritical state (12) or as dilute ammonium hydroxide (0.1-28%) (13-14). Ammonia recycled percolation (ARP) (15) and AFEX pretreatment are leading ammonia based biomass pretreatment technologies. However, most current pretreatment processes rely on pretreating the biomass using a largely liquid pretreatment medium (with varying ammonia concentrations, 0.1-99%).
In particular, AFEX represents a unique and effective pretreatment for biologically converting lignocellulosic biomass to ethanol (Dale, B. E., 1986. U.S. Pat. No. 5,037,663; Dale, B. E., 1991. U.S. Pat. No. 4,600,590; Alizadeh, H., F. Teymouri, T. I. Gilbert, B. E. Dale, 2005. Pretreatment of Switchgrass by Ammonia Fiber Explosion. Applied Biochemistry and Biotechnology, 121-124:1133-1141; Dale, B. E., 1991. U.S. Pat. No. 4,600,590; Dale, B. E., 1986. U.S. Pat. No. 5,037,663). In AFEX pretreatment, lignocellulosic biomass is exposed to concentrated ammonia at elevated pressures sufficient to maintain ammonia in a liquid phase at moderate temperatures (e.g. around 100° C.). Residence times in the AFEX reactor are generally less than 30 minutes. To terminate the AFEX reaction, the pretreated biomass is depressurized (flashed). The AFEX process is not and has never been limited to the application of anhydrous ammonia with AFEX. Some water is always initially present with the biomass and sometimes water is added to the biomass, so that any anhydrous ammonia is immediately converted into a concentrated ammonia water mixture on beginning the AFEX treatment. However, a detailed exploration of how ammonia and water are best combined with each other and with the biomass to achieve effective pretreatment has never been performed.
Recovery of ammonia used in AFEX pretreatment is a key objective when integrating AFEX into a broader biomass conversion process design. The existing ammonia recovery design (Eggeman, T. 2001. Ammonia Fiber Explosion Pretreatment for Bioethanol Production, National Renewable Energy Laboratory (NREL) Subcontract No. LCO-1-31055-01), which is depicted in FIG. 1, calls for compressing ammonia, which is vaporized as a result of the flash operation, and separating ammonia that remains in contact with the pretreated solids via evaporation in a dryer. The resulting vapor, which also contains water, is then delivered to a distillation column to concentrate the ammonia. The ammonia from the column is pumped up to pressure and, together with the compressed flash ammonia, is recycled to the AFEX reactor. FIG. 1 shows the existing ammonia recovery approach.
FIG. 1 shows the prior art system 10 including a closed AFEX reactor vessel 12 into which biomass, water and ammonia are introduced under pressure. Valve V1 is used to release pressure from the vessel 12. The treated biomass is transferred to a heated dryer 14. The dried biomass is transferred out of the dryer 14 for subsequent treatment. Ammonia from the dryer 14 is condensed by condenser 22 and sent to slurry column 16. Water is removed and condensed by condenser 18. Ammonia is condensed in condenser 20 and recycled to the vessel 12. Ammonia gas is pressurized in a compressor 24, condensed and recycled into vessel 12.
In AFEX, anhydrous liquid ammonia is used to pretreat the biomass at relatively low temperatures (70-180° C.), intermediate residence times (15-45 min), low moisture (10-200% on dwb), and higher ammonia loading (1:1-3:1, wt of ammonia/wt of biomass). During conventional AFEX, due to gravity, the liquid ammonia flows to the bottom of the reactor. Some amount of the liquid reacts with water and forms ammonium hydroxide, while the remaining liquid is converted to gaseous ammonia (depending on the thermodynamic gas-liquid state within the reactor). Since biomass is a poor conductor of heat, it takes a longer residence time (typically 15-45 min) to achieve the desired temperature throughout the reactor. Mixing and uniform pretreatment during AFEX is a significant problem in the absence of a suitable impeller. Mixing solid slurries using propellers and helical impellers is energy intensive and not very effective in reducing mass and heat transfer limitations. In other words, the biomass which is in contact with ammonium hydroxide and is suitably preheated (i.e., typically biomass close to the walls or at the bottom of the reactor) is pretreated under better conditions, as compared to the bulk of the biomass in the reactor. Another major economic hurdle to the AFEX process is the expensive recovery step, where ammonia needs to be recovered after pretreatment as a gas, recompressed, separated from water and reused as anhydrous liquid ammonia. Also, it is difficult to conduct AFEX in a continuous manner using pressurized liquid ammonia as the pretreatment chemical. The expansive release of ammonia at the end of AFEX pretreatment is energy intensive, generating gaseous ammonia-water mixtures that could make it commercially prohibitive. Supercritical ammonia based pretreatments are much more energy intensive than AFEX, making them an economically less viable option.
It is obvious to one skilled in the art that the ammonia pretreatment and recovery processes generate ammonia and water mixtures of differing phases (gaseous and liquid), compositions and temperatures. These resulting ammonia and water mixtures can therefore potentially be combined with each other and with new biomass to be treated in many different compositions and phases (gas and liquid).
The problem is that some of these potential ammonia and water treatment processes may either produce relatively low biomass reactivity and/or may require large amounts of liquid ammonia or ammonium hydroxide solutions. The most effective approaches to combine recycled ammonia and water of different compositions and phases to produce a highly reactive biomass are not well-understood. The optimal order of addition of water, ammonia and ammonia-water mixtures, and their relative amounts, temperatures and concentrations, has not been sufficiently defined so as to produce acceptable biomass reactivity. Furthermore, methods for maintaining ammonia in effective contact with the biomass, so as to reduce the total amount of ammonia required, have not been described.
Examples of previous ammonia pretreatment processes are ARP and dilute ammonium hydroxide. These processes include: high pretreatment temperature (150-180° C.), long residence time (30-120 min), high pressure liquid recycle, separation of biomass into solid and liquid fraction (by separating hemicellulose and lignin from cellulose into liquid fraction), low solids loading, and neutralization and/or recovery needed for downstream processing. Traditionally used gaseous ammoniation includes long residence time (several hours to weeks), and is expensive and inconvenient to scale-up.